The present invention relates to processes for fabricating structural units from ordered titanium-aluminum base alloys.
Pure titanium is relatively soft, weak and extremely ductile. Through additions of other elements, the base metal is converted to an engineering material having unique characteristics, including high strength and stiffness, corrosion resistance and usable ductility, coupled with low density.
Titanium is allotropic. In unalloyed titanium, up to about 785.degree. C., the atoms arrange themselves in a hexagonal close-packed crystal array called alpha phase. When titanium is heated above the transition temperature (beta-transus) of about 785.degree. C., the atoms rearrange into a body-centered cubic structure called beta phase. The addition of other elements to a titanium base will generally favor one or the other of the alpha or beta forms and will increase or decrease the beta-transus temperature.
Titanium-aluminum base alloys containing about 10 to 50 atomic percent Al and about 80 to 50 atomic percent Ti in addition to other alloying elements have been recognized for some time. These alloys are ordered and divided into two major groups: the alpha-2 alloys based on the intermetallic compound Ti.sub.3 Al, and the gamma alloys based on the intermetallic compound TiAl. There also exists a class of alloys containing a mixture of the alpha-2 and beta phases. These alloys are referred to as titanium aluminides and have good high temperature strength and oxidation and creep resistance, but are relatively brittle and difficult to process and fabricate at room temperature.
Titanium alloys are widely used in aerospace applications due to the characteristics listed previously. Titanium alloys have been fabricated into useful shapes by forging, extrusion, rolling, drawing, casting and powder metallurgy. In recent years, a fabrication technique known as superplastic forming (SPF), with or without concurrent diffusion bonding (DB), has achieved a certain prominence. This process makes it possible to form titanium alloys in a simple manner with significant reduction in parts such as fasteners, thereby permitting the fabrication of airframe and engine structures with significant cost and weight reduction.
The production of titanium aluminide SPF/DB components requires sheets and foils with uniform and fine grain structure. Such fine grain structure can require multi-step and expensive thermomechanical and/or thermochemical processing to convert ingot material into sheets and foils.
Attempts to roll titanium aluminides into sheetstock thinner than about 0.5 mm have proved to be very difficult and expensive. Consequently, researchers wishing to employ these alloys have often resorted to chemical milling or grinding of sheetstock in order to provide foil of desired thickness, typically about 0.1 to 0.3 mm, thereby greatly increasing material cost.
Sandwich panels, such as lightweight core laminate panels, have the advantage over conventional construction materials that they combine a low weight per unit area with exceptional flexural rigidity and good vibration damping. Such panels conventionally consist of two relatively thin outer covering layers of a hard, firm and rigid material. These two covering layers are joined together by a relatively thick core which consists of a light and less rigid material. The bond between the core and the covering layers must be sufficiently strong that no detachment of the covering layer from the core occurs upon application of a force.
Accordingly, it is an object of the present invention to provide a process for producing titanium aluminide SPF/DB components.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art from a reading of the following detailed disclosure.